The present invention relates broadly to a surveillance radar apparatus, and in particular to a radar capability display apparatus.
The state of the art of radar display apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 4,053,890 issued to Woodson et al on 11 October 1977;
U.S. Pat. No. 4,319,247 issued to Warren on 9 March 1982;
U.S. Pat. No. 4,499,469 issued to Kesterson on 12 February 1985; and
U.S. Pat. No. 4,723,124 issued to Boles on 2 February 1988.
The Woodson et al patent describes an internal calibration system utilizing a pulse-generation means which produces RF pulses that are injected into the radar receiver. A range control means and an attenuation control means selectively vary the range and the signal strength of the RF pulses in either a manual or an automatic sequence. A display and evaluation means determines the signal strength at which a radar tracking lock is established and compares that signal strength with a standard to provide a indication of receiver operability.
The Warren patent is directed to a testing device for simulating targets at different ranges. The testing device includes a small aerial arranged to sample a transmitted pulse. This sample is fed to a circuit which imposes successive delays on the sample and feeds its back to the aerial at successive different times.
The successive delays are produced by a delay line and a feed-back loop containing an amplifier. The circuit also includes a frequency shifter which simulates a Doppler shift.
The Kesterson patent discloses a radar system testing apparatus wherein a signal from the radar representing the elevation position of the antenna is inputted to an analog comparator circuit for determining when the antenna position is within an adjustable specification window, thereby causing the invention to generate a simulated target echo of variable range when the invention receives the proper triggering signal.
The Boles patent discusses a synthetic aperture radar apparatus which includes the method and apparatus for ship classification by providing real-time high resolution synthetic aperture radar imagery of a translating ship under the influence of rotational motions from sea state conditions.
In a typical surveillance radar, the signal strength of the echo from a given reflecting object is a function of the transmitter power, antenna size, atmospheric properties, size of the reflecting object, distance (range) to the reflecting object, and other variables. Whether this signal can be detected and will provide a certain indication that the object is truly present at the location that is determined from the time delay of the echo and the direction of the antenna beam when the echo is observed, depends on the relative power level of the signal and the power level of the interference. Typically, the echo signal needs to be ten to one hundred times more powerful (depending on various parameters of the radar operation) than the interference for reliable detection to occur. The signal from a given type of target gets weaker as the range increases, so the power level of the interference determines the maximum range at which a target of that type can be reliably detected.
Many radars do, in fact, measure the power level of the interference in their receivers for the purpose of setting a threshold with which the received signal for each range is compared to determine whether a target should be declared to be present at that range. This tends to keep the false alarm rate of the system at a constant value as the interference changes, so the operation is referred to as constant false alarm rate, or CFAR, detection. For purposes of a radar capability display, the constant false alarm rate threshold would be measured quantitatively, and equations from radar theory would be evaluated to determine whether a target of the specified type could be detected at certain ranges. The maximum range determined thereby is plotted on the radar capability display.
While the above-cited references are instructive, there still remains a need to provide a radar that could measure the power level of the interference in its receiver, then it could compute and plot the maximum range at which a target of the specified type could be reliably detected. The effects of signal attenuation due to rain and/or pulsed interference will also be accounted for. The present invention is intended to satisfy that need.